Wavelength Division Multiplexing (WDM) is a widely used technique for transmission of optical signals through optical fibers in optical communication systems. Basically, in a WDM transmission system, a plurality of wavelengths of optical signals (or channels) are transported simultaneously over one fiber without interaction.
One problem associated with the use of WDM is the so-called chromatic dispersion. This problems arises because, in general terms, each wavelength component of a signal travels through the optical fiber at a slightly different speed, resulting in the broadening of the pulse at the arrival point. It is desired that the impact of chromatic dispersion is maintained, as much as possible, at a minimum level.
Therefore it may be argued that the performance of the WDM transmission systems is highly dependent on the impact of chromatic dispersion produced on the transmission line. In particular, it turns out that when relatively high channel bit rates, such as 10 Gb/s or larger are used, more sever impacts of the chromatic dispersion are produced.
In this regard, the impact of the chromatic dispersion mapping on the non-linear threshold, as well as the transmitter/receiver tolerance to the cumulated chromatic dispersion are of concern. Non-linear threshold is a pre-determined level of optical signal power beyond which nonlinear optical effects occur in optical fibers which may impact the signal transmission performance.
At present, specific dispersion mappings are used for systems already deployed in order to minimize the performance impairments. These mappings usually depend on the type of the optical fiber (examples of such fiber types are the known standard compliant G.652 or G.655) used in the transmission line, as well as on the characteristics of the transmitter/receiver interfaces. Example of such interface dependent characteristics are the chirp of the modulator or the performance of electrical regeneration.
A widely used approach is the compensation of the chromatic dispersion of the line fiber by means of spools of fiber having the opposite dispersion characteristics. The effect of opposite dispersion in this case is used for the desired compensation. Compensating spools of fiber may be located at the output of the transmitters (pre-compensation), in an intermediate point of the transmission line (in-line compensation) or at the input of the receivers (post-compensation).
FIG. 1, schematically shows a WDM transmission system 100 using the compensating spools solution. As shown in the figure, transmitters 101 coupled to a multiplexer 102 are shown at the transmission side and receivers 109 coupled to a de-multiplexer 108 are shown at the receiving side. The optical transmission line is represented by reference numerals 104 and 106. The optical signal generated at the transmission side is output from the multiplexer 102 and is fed into an optical amplifier 103. The optical amplifier 103 is coupled to a spool of fiber 103a which has partly opposite dispersion characteristics as compared to those of the optical transmission line 104, therefore partial dispersion compensation is provided by the spool of fiber 103a. As this dispersion compensation is provided at the starting point of the transmission, it is usually referred to as pre-compensation. A similar dispersion compensation is provided in one or more intermediate points of the transmission system, generally referred to as in-line compensation. This is represented in the figure by the optical amplifier 105 and the spool fiber 105a which has dispersion characteristics opposite to those of the transmission line fiber 104. Finally, another similar compensation is provided at the reception point of the transmission line. This is represented by the optical amplifier 107 and the spool of fiber 107a. This compensation is referred to as post-compensation.
A large part of the system performance is dominated by the amount of pre-compensation, in-line compensation and post-compensation. In order to reach optimal performances, this amount must be adjusted depending on the fiber characteristics, the transmitter/receiver characteristics and the channel power levels.
In certain solutions use is made of a tunable device placed at the input of the optical receiver in order to finely adjust the cumulated dispersion, thus reducing the need for a perfect in-line compensation. Such device is generally called Tunable Dispersion Compensating Module (TDCM) and may be used in order to optimize the Bit Error Ratio (BER) of the system, either manually or automatically when Forward Error Correction (FEC) is implemented. It is to be noted that optical compensation can be replaced or enhanced with electrical compensation within the receiver (e.g. adaptive receiver).
The above solution provides satisfactory results in eliminating the difference of cumulated dispersion between two channels having different wavelengths. In particular, it contributes to reducing the need for a perfect in-line compensation, which is very difficult or in cases even impossible for some fiber types, especially for G.653 or G.655 types.
The above solution also provides certain improvement in compensation when a change occurs in the level of the signal within the system; for example after adding or dropping of channels or when the power of an amplifier is changed. However, the above solution is not suitable for the case of Raman distributed systems with co-pumping. Raman distributed co-pumping is a widely used technique in order to increase the power of an incoming optical signal. A high-power optical beam is pumped into the transmission fiber in the same direction of propagation as that of the optical signal whereby the signal is amplified as a result of interactions between the fiber material and the pump photons.
In systems using Raman distributed co-pumping, the channel power depends on the Raman gain, therefore the dispersion mapping should be modified when the Raman gain is changed.
It is therefore desired to provide a solution for automatic tuning of chromatic dispersion compensation for a WDM transmission system when Raman pumping is used such that the drawbacks or difficulties of the known solutions as mentioned above are overcome.